Transmitters and receivers in typical high-speed digital communication systems communicate data as a series of symbols, each symbol representing a different logical value for a time period called a “unit interval,” or “UI.” For a 2-PAM signal, each symbol represents a single binary “bit” that represents either a logic one as a relatively high voltage or a logic zero as a relatively low voltage. Other encoding schemes also exist, including for example, schemes that transmit more than two possible logic values in any given UI, that encode logic values as high-to-low or low-to-high transitions, or that otherwise generate a signal based upon one or more bit values. A transmitter can thus convey data as bit patterns expressed as a voltage signal that transitions between relatively different voltage levels. A receiver can recover the bit patterns, and therefore the original data, by comparing the voltage signal against a suitable reference voltage to distinguish voltage levels during each UI.
Transmitters draw current from a power supply to express voltage levels and to transition between them. However, power supplies are imperfect. For example, the lines and pads used to convey supply current exhibit parasitic resistive, inductive, and capacitive impedances. Unfortunately, this impedance and the data-dependent supply current together cause the supply voltage to fluctuate. The reference voltage employed by the receiver can also be affected. The resulting supply and reference noise effect signal integrity and therefore limit performance.
Many systems support higher data rates by transmitting multiple data streams in parallel. For example, eight data channels may transmit eight data streams in parallel to communicate eight bits per UI. Unfortunately, simultaneously transmitting and recovering multiple bits exacerbates the problems of data-dependent supply noise because supply current can vary dramatically between UIs. The resulting problem is referred to by those of skill in the art as simultaneous switching noise, or SSN. Such instability can introduce significant noise in supply and reference voltages, and thus adversely impact performance.